The invention relates to polyurethane elastomers having a defined node density, a process for their preparation with specific polyetherester polyols, and use of such elastomers, in particular, for the production of microcellular and solid polyurethane elastomer components.
Processes for preparing polyurethanes which comprise in the so-called soft segment both polyether groups and polyester groups simultaneously have been described in the prior art.
In the process described in Plominska-Michalak, B.; Lisoska, R.; Balas, A. Journal of Elastomers and Plastics (26) 1994, pages 327-334, a polyether-based NCO prepolymer is reacted with a polyester polyol. In the resulting polyurethane elastomer, abrasion is reduced, the long-term flexural strength at room temperature and −15° C. is improved, and the viscosity of the NCO prepolymer is lowered by comparison with a polyester-based NCO prepolymer. However, one of the disadvantages of this process is that mixing of the components becomes more difficult due to the great difference in viscosity between the reaction components. Another disadvantage of this process is the inherent risk of microphase separations in the so-called soft segment of the polyurethane elastomer, which impair the end properties.
In DE-A 199 27 188, physical mixing of polyethers and polyesters in the polyol formulation is disclosed. In this way, polyurethanes which have improved oil resistance compared with that of pure polyether polyurethanes can be obtained. The inadequate storage life of the polyol formulations is disadvantageous because the low compatibility of polyesters and polyethers brings about macroscopic demixing after a relatively short time. Users of such a system experience undesirable difficulties in terms of storage and logistics.
These disadvantages may be circumvented with the use of separation-stable polyetherester polyols which can be prepared by discontinuous synthesis processes such as: insertion in polyethers, alkoxylation of a polyester with alkylene oxide, polycondensation with alkylene oxide, two-stage and single-stage polycondensation.
In practice, however, it has been found that such polyetherester polyols do not produce polyurethanes (PUs) having generally good long-term flexural strength, particularly if they have been exposed to hydrolytic ageing.
In U.S. Pat. No. 5,436,314 (insertion in polyethers), carboxylic acids or carboxylic acid anhydrides are reacted with polyether polyols in the presence of strong Brönsted acids, and polyetherester polyols having randomly distributed ester groups are obtained. However, these products do not have polymethylene segments of different lengths, though they contribute substantially to the good properties of many polyesters. The metal salts of strong Bronsted acids furthermore contaminate the polyetheresters and diminish the hydrolytic stability of their ester bonds to such an extent that their use in, for example, shoe soles results in inferior materials.
When alkoxylating a polyester with alkylene oxide, polyesters are first prepared and then alkoxylated with alkylene oxide. This is a widespread method which results in 3-block copolymers, the polyester-block-polyether polyols. An inherent disadvantage of the process is that the block structure of the polyether-block-polyester polyols, created with such an outlay, is not in transesterification equilibrium. For this reason, they may rearrange at elevated temperature and lose their constitutional structure. This has an undesirable effect on their storage stability.
In DE-A 198 58 104, a polyestercarboxylic acid is synthesized in the first stage from ring esters, alcohols and carboxylic acids, and in the subsequent step is alkoxylated with ethylene oxide or propylene oxide, preferably without additional catalysts. The products serve as raw materials for rigid foams. They reduce shrinkage, increase strength and reduce the tendency to crystallize. These advantages can, however, be realized only when at least one polyol component or one isocyanate has a number average functionality markedly greater than 2, which enables a highly cross-linked polyurethane system to be constructed. As is generally known in the art, microcellular elastomers having good properties such as, for example, good long-term flexural strength cannot be obtained in this way. The ring esters from which the polyester polyols of the first synthesis step are synthesized can not be prepared without increased expense, because they must first be obtained from mixtures of linear and cyclic esters by extraction or distillation, a major disadvantage of this process.
In U.S. Pat. No. 4,487,853, acid semi-ester intermediates are prepared by esterification of polyether polyols with carboxylic acid anhydrides and then ethoxylated with an amine or tin compound catalyst. The polyester-co-polyether polyols obtained are low in ester-groups and have a high proportion of terminal primary hydroxyl groups. However, it is disadvantageous that the ether groups are used in a very high excess vis-à-vis ester groups, such that the advantages of typical polyetherester polyols or polyester-block-polyether polyols are not fully realized. Dicarboxylic acid anhydrides, which are generally costly, must be used as educts for the synthesis of adipic acid esters, which are important raw materials for polyurethane elastomers.
Double metal cyanide catalysts are used in WO 200127185. They enable ether blocks to be started on polyesterols with few by-products and unsaturated terminal groups. The products have good miscibility with ethers and esters which the specification recommends as surface-active agents or phase promoters. However, it is known that polyethers having a large number of terminal primary hydroxyl groups can not be prepared with double metal cyanide catalysts, because the ethylene oxide polymerization starts on a small number of hydroxyl groups where it constructs high molecular weight polyethylene oxide units. For this reason, the polyester-block-polyether polyols disclosed in WO 200127185 have only limited use in polyol formulations, specifically when polyols having a majority of secondary hydroxyl groups of low reactivity are suitable for the application. This limitation is a major disadvantage for many applications.
In DE-A 21 10 278 (polycondensation with alkylene oxide), polyether polyol, carboxylic acid anhydride and alkylene oxide react in a one-pot process to give polyetherester polyol having randomly distributed polyether units. Because of the nature of the process, the alkylene oxides form only derivatized dimethylene bridges. Longer carbon bridges, such as those used in butanediol esters or hexanediol esters, are lacking. Costly adipic acid anhydride must be used in this process.
In DE-A 34 37 915 (two-stage polycondensation), a polyether polyol is reacted with a carboxylic acid or a carboxylic acid anhydride or a carboxylic acid ester to obtain a polyestercarboxylic acid which in a second step is reacted with aliphatic alcohols to give the actual polyetherester polyol. Disadvantages of this process are, on the one hand, the multi-stage process and, on the other, the costly carboxylic acid derivatives. A similar process is described in DE-A 34 37 915. In this process, a traditional polyester polycarboxylic acid is not constructed from polyether polyols, but is reacted with polyether polyols and aliphatic alkanols.
According to EP-A 0 601 470 (single-step polycondensation), polycarboxylic acids, alkanediol mixtures and polyether polyols are condensed to give randomly distributed polyetherester polyols having a ratio of ether groups to ester groups in the polyetherester polyol of from 0.3 to 1.5. The particular advantage of this process resides in the fact that polyurethane flexible foams having reduced fogging can be prepared by reacting these polyetherester polyols with polyisocyanates.